Discovery of 5-substituted-N-arylpyridazinones as inhibitors of p38 MAP kinase

Article abstract of DOI:10.1016/j.bmcl.2010.03.088

The synthesis, structure-activity relationship and modeling of a series of 5-substituted-N-aryl pyridazinone based p38α inhibitors are described. In comparing the series to the similar N-aryl pyridinone series, it was found that the pyridazinones maintain

Full text of DOI:10.1016/j.bmcl.2010.03.088

Bioorganic & Medicinal Chemistry Letters 20 (2010) 3146–3149
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of 5-substituted-N-arylpyridazinones as inhibitors of p38 MAP kinase
à
*
Kevin D. Jerome , Michael E. Hepperle , John K. Walker, Li Xing, Rajesh V. Devraj, Alan G. Benson ,
John E. Baldus, Shaun R. Selness
Pfizer Global Research and Development, St. Louis Laboratories, 700 Chesterfield Pkwy. W., Chesterfield, MO 63017, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis, structure–activity relationship and modeling of a series of 5-substituted-N-aryl pyridazi-
Received 28 January 2010
Revised 24 March 2010
Accepted 26 March 2010
Available online 31 March 2010
none based p38a inhibitors are described. In comparing the series to the similar N-aryl pyridinone series,
it was found that the pyridazinones maintained a weaker interaction to the p38 enzyme, and therefore
showed generally weaker binding than the pyridinones.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
p38
Pyridazinones
Kinase
Inhibition of the p38
a
mitogen-activated protein (MAP) kinase
We herein describe the discovery of a series of 5-substituted-N-
aryl pyridazinones as potent p38 inhibitors, derived from the
N-benzyl pyridinone high-throughput screening hit, SC-25028, 4.
The evolution of the pyridinone template is shown in Figure 2.
has been the focus of much pharmaceutical and academic research
in recent years.1 p38 has been pursued as a kinase target for such
diseases as rheumatoid arthritis (RA), Crohn’s disease, inflamma-
tory bowel syndrome (IBS), and psoriasis.2 p38 is a serine/threo-
The enzyme activity of 4 against p38a was modest at 0.68 lM.
nine dual specificity kinase that has four known isoforms (
a, b,
c,
However, evaluation of 4 against a panel of 48 kinases revealed
and d), with their expression varying among cell types of the im-
that the compound was selective for p38 (less than 30% inhibition
mune system. p38
a
is the chief isoform implicated in inflamma-
at 10 lM against the entire panel). Metabolic stability of 4 was rec-
tory disease.3
ognized as the primary liability as revealed by its incubation with
human liver microsomes (52% parent remaining after 45 min of
incubation). Previous reports have shown optimizations of 4 in
the pyridinone template to develop N-benzyl inhibitors8 such as
5. Further optimization of this template led to N-aryl pyridinone
inhibitors, culminating in the clinical candidate PHA-797804,9 6.
In conjunction with our efforts in the N-aryl pyridinone scaffold,
One current treatment option for these inflammatory diseases
is with biological agents known for successfully shutting down tu-
mor necrosis factor
support that selective inhibition of p38
TNF- , as well as interleukin-1b (IL-1b) and other cytokine produc-
tion. Because TNF and IL-1b have been implicated as key cyto-
kines in inflammatory diseases, it is anticipated that a small
molecule p38 MAP kinase inhibitor should provide therapeutic
a (TNF-a
function.4 Preclinical studies strongly
a
can effectively modulate
a
a
we initiated the investigation of N-aryl pyridazinones as p38
a
a
inhibitors ( Fig. 3), based on the similarity in structural overlay
value for the treatment of such inflammatory conditions.
Some historical p38 inhibitors are shown in Figure 1. Research-
ers at Vertex have previously demonstrated both potency and
and similar presentation of suitable binding motifs. In addition to
selectivity versus p38a
kinase with inhibitors such as VX-745, 1.5
F
F
S
R1
R2
More recently, Merck reported a series of potent 6-imidazopyr-
idyl-2-arylpyridazin-3-one p38 inhibitors 2,6 followed by a series
of 6-arylamino-2-arylpyridazin-3-ones 37 that also showed good
N
R1
N
N
N
N
O
R2
Cl
Cl
N
N
N
N
Cl
p38a potency and selectivity versus p38c, p38d, JNKs and ERKs.
N
O
O
Cl
* Corresponding author. Tel.: +1 636 247 8655; fax: +1 636 247 0966.
E-mail address: jerome52@sbcglobal.net (K.D. Jerome).
Present address: Nycomed GmbH, Byk-Gulden-Str. 2/F21.311, D-78467 Konstanz,
Germany.
Merck
Merck
VX-745
3
1
2
à
Present address: Pharmaceutical Products, Covidien, 675 McDonnell Blvd.,
Figure 1. Historical p38 inhibitors.
Hazelwood, MO 63042, USA.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.03.088

K. D. Jerome et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3146–3149
3147
F
O
F
F
N
O
O
Br
N
O
N
F
N
Br
Br
O
O
N
H
O
5
4
6
SC-25028
PHA-745052
p38α IC50 = 0.059 μM
PHA-797804
p38α Ki = 0.0058 μM
p38α IC50 = 0.68 μM
Figure 2. Pyridinone p38a inhibitors based on SC-25028.
R2
O
R2
O
N
70 °C to form an N-2 substituted, 4,5-dibromo pyridazinone 8.11
Nucleophiles were then used to displace the C-5 bromide to form
9. For amines, Cs2CO3 in DMF were the standard conditions. For
alcohols, either Cs2CO3 in DMF or DBU in methylene chloride could
be used. This displacement occurred exclusively at C-5, leaving the
C-4 bromide intact.12 In method B, 7 was condensed with THP
hydrazine to form the N-2 THP, 4,5-dibromo pyridazinone 10. A
similar nucleophile displacement to that described above formed
11. Deprotection, followed by reaction with alkyl and aryl bro-
mides gave 9.
N
N
R1
Br
R1
Br
O
O
Figure 3. Structure Based Drug Design of N-aryl pyridazinones from N-aryl
pyridinones.
extending our intellectual property position, we also anticipated a
reduction in log P with the pyridazinones, which was expected to
increase inherent solubility of the series. Using the pyridazinone
core also removes the C-6 methyl of the pyridinone core and the
accompanying chirality due to stable atropisomers formed by the
aryl pyridinone analogs,9 such as 6, which is the single aS atrop-
isomer. Initial analogs were based on the initial lead 4 and the
SAR gleaned from the development of 5.
Two generalized syntheses of the pyridazinone template are
shown in Scheme 1.10 Method A allows for the diversification of
the C-5 position with a fixed N-2, and method B allows for the
diversification of the N-2 position, with a fixed C-5. In method A,
mucobromic acid 7 was condensed with hydrazines in 6 N HCl at
Initially, we investigated pyridazinone mimics of the screening
hit 4. The direct pyridazinone mimic 13, where Z = O and R1,
R2 = benzyl, was found to be inactive against p38
a (Table 1). Mod-
ifying the R1 group to a phenyl 14 gave modest activity. Modifica-
tion of R2 to a 2,4-difluorobenzyloxy 15, which was known to be an
optimal group in the N-aryl pyridinone series, showed a 10-fold in-
crease in potency to 2.7 lM against p38a. Another dramatic
increase in potency was seen when R1 was replaced with 2,6-
dichlorophenyl, as found in 1, which is known to impart a desired
orthogonality to the R1 phenyl ring.
Method A
R1
O
H2N N
H
R2-ZH
Br
Br
Br
Br
Z
R2
Br
N
N
N
N
OH
R1
R1
6N HCl, 70ºC
Cs2CO3, DMF
O
O
O
or
7
8
9
DBU, CH2Cl2
Z = O, NH
Method B
THP
O
H2N N
Br
Br
R2-ZH
2Z
Br
H
N
R
N
OH
N
N
THP
Br
THP
Br
O
6N HCl, 70ºC
Cs2CO3, DMF
O
O
or
10
11
7
DBU, CH2Cl2
Z = O, NH
4M HCl
in 1,4-dioxane
R1 Br
Z
R2
Br
N
N
Z
R2
N
NH
R1
Br
Cs2CO3, DMF
O
O
9
12
Z = O, N
Scheme 1. Alternate syntheses of the pyridazinone scaffold.

3148
K. D. Jerome et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3146–3149
Table 1
Based on SAR gleaned from the N-aryl pyridinone series, the
p38a Potency of selected 4-bromo-disubstituted pyridazinones
corresponding analog to the clinical candidate 6 was synthesized
in the pyridazinone series (Scheme 2). The synthesis of 32 pre-
ceded by method A, as shown in Scheme 1. The methyl ester of
32 was hydrolyzed and 33 was then formed via an amide bond
coupling with methylamine. Compounds 32 and 33 were both ac-
Compd
Method
Z
R1
R2
p38aa IC50
(lM)
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
A
A
A
A
A
A
A
A
A
A
A
A
A
A
B
B
O
O
O
O
N
O
N
O
N
O
O
O
O
O
O
O
Benzyl
Phenyl
Phenyl
Benzyl
Benzyl
2,4-DiF bz
Benzyl
Benzyl
Phenethyl
Phenethyl
4-F bz
>100
24.4
2.70
1.39
5.72
3.14
3.81
1.32
12.3
0.16
0.54
2.07
0.47
1.01
0.25
0.59
2,6-DiClPh
2,6-DiClPh
2,6-DiClPh
2,6-DiClPh
2,6-DiClPh
2,6-DiClPh
2,6-DiClPh
2,6-DiClPh
2,6-DiClPh
2,6-DiClPh
2,6-DiClPh
2,6-DiCl-4-pyr
2,6-DiFPh
tive against p38a, with 32 showing an order of magnitude increase
in potency over 22.
Based on modeling, the binding of pyridazinone compounds are
consistent with that of the pyridinone series. Compound 32 was
a
binding pocket using AGDOCK ( Fig. 4).13
4-F bz
docked into the p38
The protein model is derived from the 6–p38
2,4-DiF bz
2,4,6-TriF bz
3,4-DiF bz
2,3,4-TriF bz
2-F-4-ClPh
2,4-DiF bz
2,4-DiF bz
a
binary complex,9
with 2A cushions in each dimension of the docking box and a max-
imum energy barrier of 25 kcal/mol. Consistent with the pyridi-
none binding, the 2,4-diflouorobenzyloxy moiety of 32 occupies
the unique p38 encapsulated hydrophobic pocket, defined by
Thr106 as the gate keeping residue. The pyridazinone carbonyl
forms bifurcated hydrogen bonds with the kinase hinge, in which
Gly110 undergoes a peptide flip that is prohibited by other amino
acids bearing a side chain. The orientation of the N-methyl benz-
amide of 6 is designated by the polar interaction of the amide
NH and the backbone of Gly110. However, modeling of 32 suggests
that the hydrophobic contact between the methyl amide and the
side chain of Leu108 (ꢀ4 A) disrupts the binding conformation.
In particular the sevenfold activity loss of the N-methyl amide ana-
log 33 over the methyl ester 32 appears to support this binding
model.
a
All analogs were >200
l
M versus JNK2.
With a suitable R1 group in place, the focus was shifted to fur-
ther optimization of R2. As analogs 16–26 show, 2,4-difluoroben-
zyloxy was found to be optimal in the pyridazinone series as
well, with 22 showing 160 nM inhibition of p38
17, 19 and 21 were prepared to evaluate the role of the C-5 linkage.
a. Compounds
In all cases, this change showed a decrease in p38a inhibition.
Further optimization was attempted at R1, as shown in 27 and
28, but failed to improve potency.
In general, the pyridazinone series was found to be less potent
than the pyridinone series. Since orthogonality between the N-aryl
and the core is required by target binding, the removal of the 6-
methyl when comparing the pyridinone template to the pyridazi-
none introduces conformational entropy loss from the free-ligand
state to the bound state for the pyridazinone series. Also, the elec-
tronic properties of the pyridazinone carbonyl oxygen, the accep-
tor of the critical bifurcated hydrogen bond, are influenced by
the atomic composition of the aryl ring. Density functional theory
(DFT) calculations performed on the cores reveal that the
electrostatic potential (ESP) charge of the oxygen atom decreases
from À0.50 for pyridazinone to À0.56 for pyridinone, a factor of
12%.14
In an attempt to minimize the molecular weight of the analogs
and thereby increase the inherent solubility of the series, changes
were made at C-4 in an attempt to discover a suitable Br replace-
ment. This group at C-4 occupies a shallow lipophilic pocket adja-
cent to the deep hydrophobic selectivity pocket as defined by
Thr106. This group also favors proper orientation about the ether
linkage to optimize the benzoxy group’s interaction with the selec-
tivity pocket. The optimal C-4 group would be big enough to cause
the C-5 benzyl group to swing into the critical hydrophobic selec-
tivity pocket, yet small enough to still fit into its own shallow
lipophilic pocket. Despite synthesizing several analogs with alter-
nate C-4 substituents, such as Me, Cl and I, Br was found to be
optimal.
NH2
HN
O
O
29
O
Br
Br
Br
Br
Br
MeOH
N
N
N
OH
N
Br
O
4N HCl in
1,4-dioxane
O
O
6N HCl, 70ºC
7
30
OH
O
31
O
O
F
OH
F
Cs2CO3, DMF
F
F
O
1) 2.5 N NaOH
THF/H2O
O
N
N
N
F
N
F
Br
Br
O
2) MeNH2
EDCI, HOBT,
TEA, CH2Cl2
O
N
O
O
O
H
32
33
p38α IC50 = 0.10 μM
p38α IC50 = 0.015 μM
Scheme 2. Synthesis of the pyridazinone analog of 6.

K. D. Jerome et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3146–3149
3149
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Figure 4. Pyridazinone 32 modeled into the p38a binding pocket.
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weaker p38a enzyme activity in the pyridazinone series verses
the pyridinone series.
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**
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